1. Field of the Invention
The present invention relates to a brushless motor which is installed as a drive member of a compact disc player, a tape player or the like.
2. Description of the Prior Art
FIG. 4 is a sectional view of a brushless motor in the prior art illustrating its schematic structure.
Numeral 1 designates a rotary shaft. The rotary shaft 1 is rotatably supported by a thrust bearing 2 and a radial bearing 3. A rotor yoke 4 is fixed to the rotary shaft 1, and a ring-shaped rotor magnet 5 is fixed to a lower surface of the rotor yoke 4. A coil substrate 7 is fixed on the stationary plate 6. The coil substrate 7 is a flexible substrate. A plurality of stator coils 8 are fixed on an upper surface of the coil substrate 7. The stator coils 8 are wound in nearly square form and arranged at regular intervals as shown in FIG. 5. Current is supplied to each stator coil 8 by a lead pattern formed on the coil substrate 7.
As shown in FIG. 6, the rotor magnet 5 comprises a main magnetizing member 5a for rotation driving, formed on a flat plane portion opposed to the stationary plate 6. In the main magnetizing member 5a, N-poles and S-poles are alternately arranged in the circumferential direction and magnetized. The stator coil 8 is constituted so that current flows in the radial direction at linear parts 8a and 8b. Electromagnetic force in the rotational direction acts on the rotor magnet 5 according to the current flowing through the linear parts 8a and 8b and the polarity of the main magnetizing member 5a opposed to the current.
As shown in FIG. 7, an FG, (frequency generating) magnetizing member 5b for detecting the rotational speed is formed on a circumferential surface of the rotor magnet 5. In the FG magnetizing member 5b, N-poles and S-poles are alternately formed in the circumferential direction at a shorter pitch than that of the main magnetizing member 5a. A pair of PG magnetizing members 5c and 5d for detecting the rotational position are formed also on the circumferential surface of the rotor magnet 5. The PG magnetizing members 5c and 5d have different polarity N and S, and are arranged at a spacing of 180.degree..
As shown in FIG. 4 and FIG. 7, a detecting substrate 9 surrounds the periphery of the rotor magnet 5. FIG. 8 shows the detecting substrate 9 in a planar view. The detecting substrate 9 is provided with an FG pattern 10 for detecting the rotational speed and a PG pattern 11 for detecting the rotational position. The FG pattern 10 is opposed to the FG magnetizing member 5b of the rotor magnet 5, and a plurality of detecting parts 10a are formed continuously at the same pitch as the magnetizing pitch of the FG magnetizing member 5b. If the rotor magnet 5 is rotated, pulses are outputted from the FG pattern 10 corresponding to the magnetic poles of the FG magnetizing member 5b. The PG pattern 11 is formed at one position in a U-like shape. The PG pattern 11 is oppossed to the position where the PG magnetizing members 5c, 5d pass. If the rotor magnet 5 is rotated and the PG magnetizing member 5c or 5d passes, pulses corresponding to the polarity of N or S are outputted from the PG pattern 11 whereby the rotational position of the rotor magnet 5 can be recognized.
The detecting substrate 9 shown in FIG. 8 is provided with the FG lead patterns 10b and 10c connected to both ends of the FG pattern 10, and the PG lead patterns 11a and 11b connected to both ends of the PG pattern 11. The FG lead pattern 10b and the PG lead pattern 11a are connected and used as the common lead. The FG pattern 10c and the PG lead pattern 11b extend individually to the edge portion of the detecting substrate 9.
In the detecting substrate 9 shown in FIG. 8, part of the FG lead patterns 10b and 10c corresponding to the length A overlaps the passing route of the PG magnetizing members 5c and 5d of the rotor magnet 5. Consequently, every time the PG magnetizing members 5c, 5d pass the FG lead pattern 10b or 10c, electromotive force acts on the FG lead pattern 10b, 10c, whereby unwanted current flows through the FG pattern 10. Thus regular distortion as shown in FIG. 9 is produced in the rotational speed detecting pulses P taken from the FG pattern 10.